Typically, leads for implanted medical devices, such as, but is not limited to, cardiac stimulation devices, comprise an insulated sheath made of a flexible material having at least one internal electrical conductor (two conductors in the case of a bipolar lead), terminated at its distal extremity by an electrode disposed to contact tissue, for example, the wall of the myocardium in the case of a cardiac stimulation device.
The construction of the distal extremity of the lead is primarily an assembly of three elements, namely the cylindrical body (also known as "heart-side"), the stimulating electrode, and the internal conductor. The stimulation electrode is generally a porous carbon material that has the general form of a nail, with a head (stimulation surface in contact with the tissue) and an axial stem that is designed to allow connection to the internal conductor in the cylindrical body. The axial stem is typically cylindrical, although it also may be tapered. The head may be flat, convex or concave.
The internal conductor is a metallic element that is typically spiral wound inside the lead sheath. The mechanical and electrical connection between the electrode and the conductor is obtained by introducing, with force, the stem of the electrode into the spiral of the conductor, and alternately, according to a technique described in FR-A-2,616,072 and its corresponding U.S. Pat. No. 4,917,106, commonly assigned to the assignee hereof, ELA Medical, which U.S. Pat. No. 4,917,106 is incorporated herein by reference in its entirety.
The electrode and conductor are so fixed together, and the mechanical connection of the electrode to the cylindrical body is set by gluing, that is, by depositing a certain quantity of glue in an opening of the cylindrical body and by introducing into this cylindrical body opening the stem of the electrode, until the glue comes to overflow slightly in the periphery of the opening.
The technique of setting by gluing necessitates, however, a certain skill to master well the quantity of glue introduced, and to avoid particularly an overflow, especially on the active surface of the porous carbon electrode, which active surface has to be defined with precision because it is the surface electrode that fixes the polarization for the stimulation of the myocardium in the example described.
Although it is possible, in case of overflow, to wipe off the surplus of glue, the overflow can have already altered the active surface of the porous carbon electrode. Indeed, if the surplus of glue is not correctly cleaned, it is necessary, once the glue is hardened, to scratch (scrape) the surface of the electrode at this location in order to remove the glue and expose the material of the electrode. This can alter the qualities of this functional surface. During the injection of the glue, skill also is required because one also should also avoid the formation of bubbles in the glue, which bubbles could result in a bad fixation and/or a poor tightness of the connection between the electrode and the cylindrical body.
These difficulties are further complicated in the particular case of leads comprising, between the electrode and cylindrical body, a intermediary ring formed of a porous material containing an active material such as a steroid or other drug destined to be gradually distributed (or diffused) in the region of contact between the electrode and the myocardium. Here again, an overflow of glue on the functional surface of the diffusion ring creates a risk of reducing the surface available for releasing the active material, and therefore to modify the functional properties of the ring.